Estrogen stimulates expression of chicken hepatic vitellogenin II and very low-density apolipoprotein II through ER-α

Steroid hormones and their receptors play pivotal roles throughout vertebrate reproduction and development. Egg formation in avian species is a prime example. The synthesis of egg yolk proteins by the liver is highly dependent on estrogen. Two major components of the yolk protein precursors, vitellogenin II (VTG II) and very low-density apolipoprotein II (ApoVLDL II), are synthesized in the liver of hens under estrogen stimulation and are subsequently transferred via the blood to the developing oocytes. Estrogen-inducible transcription can be mediated through estrogen receptors (ERs) (ER-α and ER-β) or through G protein-coupled receptor 30 (GPR30), but the exact participation of the individual receptor is not clear. Here, we determine the relative contribution of each transduction pathway in the synthesis of VTG II and ApoVLDL II in the hepatocytes by using selective compounds that are known to specifically interact with each of the ERs and GPR30. 17β-Estradiol and propyl pyrazole triol (PPT, ER-α agonist) induced increase in VTG II and ApoVLDL II mRNA expressions in a dose-dependent manner. A high concentration of diarylpropionitrile (DPN, which preferentially motivates ER-β) slightly stimulated the expression of VTG II and ApoVLDL II mRNAs. However, G-1 (a GPR30 agonist) failed to display any stimulating role. Methyl-piperidino-pyrazole (a highly selective ER-α antagonist) fully blocked the expression of both yolk precursors, which were upregulated by 17β-estradiol, PPT, and DPN. Considering that DPN can also provoke the action of ER-α at high concentration, this excludes the participation of ER-β and supports the role of ER-α. The aforementioned results indicate that estrogen stimulates the expression of VTG II and ApoVLDL II mRNAs predominantly through ER-α in the chicken liver.